This invention is related to a water-borne coating composition of general utility in coating metallic substrates, for example in automotive and coil coatings applications. More particularly, the coating composition is useful as an interior and exterior can coating.
The can coating industry utilizes a variety of thermosetting coating compositions designed to fulfill various requirements dictated by the intended use. Cans are usually coated on their interior by a thin coating designed to protect the metal walls from attack by food or beverage, and vice versa, to be stored therein. Such coatings should have among other properties good adhesion to the metal walls, low extractables and sorption to prevent taste adulteration, and a rapid cure rate for an economical manufacturing process.
The coatings of the prior art are thermosetting compositions often dissolved or dispersed in organic solvents. Among commonly utilized thermosetting compositions are those based on epoxy resins crosslinked with nitrogen resins, usually in an acid catalyzed process.
Increased awareness of the environmental hazards of allowing organic solvent vapors to enter the atmosphere, the desirability of a single system that can be applied not only by the more conventional techniques of spray, roller or flow coating but also by electrodeposition, and the economy resulting from the substitution of water for some or all of the solvents in a coating composition, are all factors mitigating in favor of water-borne coating compositions.
U.S. Pat. No. 2,676,166, issued Apr. 20, 1954 to V. J. Webers, shows the conversion, through the use of tertiary amines, of random low molecular weight hydrophilic copolymers of oxirane containing monomers with other ethylenically unsaturated monomers into water soluble materials. However, these polymers contain a large number of hydroxyl and quaternary ammonium groups per molecule.
In U.S. Pat. No. 3,839,252, issued Oct. 1, 1974 to J. E. Bosso et al., there are described water dispersible products of epoxy resins with amine salts. These products are designed to contain at least one unreacted 1,2-epoxy group per molecule and also contain in the resin quaternary ammonium salt groups.
The composition of this invention is an aqueous solution or dispersion of the reaction product of a terminally functional epoxy resin with a tertiary amine and a nitrogen resin crosslinking agent. These aqueous solutions or dispersions of the polymeric quaternary ammonium hydroxide and a nitrogen resin are stable both uncatalyzed or catalyzed and can be applied to metallic substrates by spray, roller, dip or flow coating or by electrodeposition at the cathode and to paper.